This invention relates to a method of making a plugged microporous film.
The term "film" is used to denote sheets of a material with a thickness of less than 2.0 mm. The film of the invention has pores which extend therethrough, that is to say, which extend from one surface of the film to the other surface for the transmission of fluids through the film, at least some of the pores containing plugging material capable of acting as a barrier to transmission of certain fluids through the pores. The pores are microscopic, that is to say, the details of their structure are discernable only by microscopic examination. Preferably the structure of the pores is so fine that discernment thereof is possible only by use of electron microscopy techniques, which can resolve details of structure below 500 nm.
Microporous films are used in a variety of applications to provide selective barriers, allowing the passage of a first material while acting as a barrier to passage of a second material. For example, such films are commonly used as gas separation membranes, reverse osmosis membranes, packaging material especially for sterilised medical equipment and as barriers to bacteria. The films may be used as filtering membranes, for example to separate solid materials from gases and liquids. Furthermore, microporous films may be used as semipermeable membranes for separating wetting fluids from non-wetting fluids; for example, a gas-saturated membrane in contact with a liquid and a gas will transmit the gas (the wetting phase) but it will not transmit the liquid (the non-wetting phase) provided that the pressure in the liquid does not exceed the liquid entry pressure for the particular combination of membrane and fluids. An application of microporous films which makes use of these semi-permeable membrane properties is to confer waterproof properties on a fabric, while allowing the fabric to breathe. When the fabric is incorporated into an article such as a garment, tent or other protective article, the article is resistant to transmission of liquid water while allowing transmission of water vapour, thereby minimising the formation of condensation on the inside of such an article.
The transmission properties of microporous films are determined principally by the material and the thickness of the film and the structure of the pores (including the dimensions and tortuosity). However, the properties of the films may be modified by the presence of a layer of another material through which any fluid passing through the pores in the film has to pass. This layer of other material can be considered as providing an additional filter. For example, there is disclosed in GB-A-2024100 a flexible layered article for use in waterproof garments which permits transmission of water vapour but is resistant to the transmission of liquid water. The article comprises a layer of porous hydrophobic material and a continuous hydrophilic layer which is attached to a face of the layer of hydrophobic material. The hydrophilic layer forms a barrier to surface tension lowering agents (such as certain constituents of perspiration) which would, if present in the layer of hydrophobic material, tend to reduce the waterproofness of that layer.
GB-1493654 discloses a process for the production of a semi-permeable membrane in which a blend of water-soluble and water-insoluble polymers is formed into a film, and a surface layer of the water-soluble polymer is crosslinked, thereby rendering that layer insoluble. The remaining soluble polymer is removed by washing with water. The resulting article consists of a semi-permeable membrane of crosslinked water-soluble polymer that has been formed integrally with a support provided by the water-insoluble polymer. In order to ensure that only a thin surface layer of the water-soluble polymer is crosslinked, crosslinking is effected by contact with a plasma. This involves maintaining the membrane in a low pressure atmosphere of a gas such as nitrogen or hydrogen for a period of 1 to 180 minutes, with an exposure of around 60 minutes often being necessary for a satisfactory crosslink density.